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THERE are few more delightful antidotes to stress than to lie on your back in warm grass and watch the clouds go by. As children, we love finding the outlines of animals and castles in the billowing shapes. As adults, there is something calming and comforting about those fluffy tufts of white drifting slowly past. Clouds are beautiful. Clouds are innocent.
With one exception. The streaky smears of cloud that criss-cross the sky in the wake of aeroplanes may look too wispy to cause any harm. But we now know that these condensation trails, or contrails, make an outsized contribution to global warming by trapping heat like a downy jacket. “They are one of the few manifestations of man-made climate change agents that you can actually observe,” says David Lee, an atmospheric scientist at Manchester Metropolitan University in the UK. As the evidence mounts to show how harmful contrails are, some engineers are reaching for an audacious solution: scrub them from the sky altogether.
Contrails are created when water condenses to form ice crystals around tiny particles of soot from aircraft exhausts. Yet there is no fundamental reason why this has to happen. Decades of experiments with spy planes, alternative engines and, most recently, with artificial intelligence have shown that it is possible to stop them forming. It won’t be easy: wiping the atmosphere clean of contrails may require nothing less than a wholesale reimagining of the traffic in our skies.
The effect of clouds on our climate is subtle because they can both reflect incoming sunlight, which has a cooling effect, and trap heat beneath them, …
